A Parade of New Planets

Alien worlds are a staple of science fiction but until just a
few months ago, they had no place in science fact. Despite
decades of earnest searches, astronomers had no direct evidence
that planets circle other stars. Then last October, Michel Mayor
and Didier Queloz of Geneva Observatory detected a planet
circling the star 51
Pegasi--and the floodgates opened. In rapid succession,
observers have reported finding at least four more planets
circling nearby stars; two of these sightings are so new that
they have not yet been formally announced in the literature.
"This is a unique era," exclaims Alan P. Boss of the Carnegie
Institution of Washington, an expert in modeling planetary
formation. "It is the most exciting thing I've seen in my
scientific career."

The Hunt is On

For now, the planet-hunting trophy goes to the team led by Geoffrey
W. Marcy and R. Paul Butler of San
Francisco State University and the University of California at
Berkeley. They helped confirm the results announced by Mayor and
Queloz, and then swiftly turned up three planets of their own.
More discoveries are surely on the way: Marcy reports that "we
see hints of planets in a lot of our data." His group has set up
a dedicated planet-search
Web site to keep up with the rapid progress of the work. A
fifth planetary detection will be reported by George Gatewood of
the University of Pittsburgh at the upcoming meeting of the American
Astronomical Society.

Nobody has actually seen the new planets; they were all
identified indirectly, by measuring the way they influenced the
motion of their parent stars. As an object orbits a star, its
gravitational pull causes the star to wobble back and forth.
That motion creates a periodic displacement, or Doppler
shift,in the spectrum of the star as seen from the earth.
Marcy and Butler, like Mayor and Queloz and several other teams,
examine spectra for the tiny displacements that could denote the
presence of planets. The
Doppler technique can reveal the orbit and the minimum mass
of an orbiting body, but no details of its nature.

Gatewood takes a somewhat different approach, one that relies on
direct observation of stellar motion. Stars are not fixed in
place; they appear to drift across the sky, though very slowly.
The same back-and-forth movement that produces Doppler shifts
also causes a star's path to appear as a zig-zag rather than a
straight line. Gatewood and his colleagues are looking--very
carefully--for those telltale wiggles. That precision
measurement of stellar positions is known as astrometry.

The success of these techniques has sent ripples of excitement
through the astronomical community and captured the public
imagination. Despite some overzealous early
statements, the newfound bodies are very unlikely to harbor
life (much less intelligent life). But they do suggest that planets are
common throughout the cosmos, raising the hope that living
things flourish somewhere among the multitude of worlds. The
planets are also quite unlike anything in our solar system,
forcing theorists to reconsider their notions of how stars and
planets form

Strange New Worlds

The planet around 51 Pegasi is
perhaps the oddest of the bunch. Its mass is at least half that
of Jupiter, and yet it orbits just seven million kilometers from
its star--less than one eighth Mercury's distance from the sun.
At such proximity, the planet's surface should be baked to a
theoretical temperature of 1,300 degree Celsius. It whizzes
around its star so quickly that its "year" is just four days
long.

Marcy and Butler recently posted a notice on their Web site of
the discovery of a somewhat similar planet circling the star
HR3522, also known as 55 Rho Cancri. (Marcy describes the
announcement as an experiment in "publishing by Internet": a way
to air a new finding before passing it through peer review.)
This object has at least four fifths the mass of Jupiter and
orbits at a distance of about 25 million kilometers.

One of the bodies identified by Marcy and Butler is a very
massive object--at least 6.5 times the mass of Jupiter. It
orbits around the star 70 Virginis in a highly eccentric, or
oval, path quite unlike those of the familiar planets. Although
some news reporters optimistically dubbed the planet
"Goldilocks," claiming it has just the right temperature for
liquid water, this heavyweight is most likely a gaseous world
lacking a solid surface on which water could collect.

The final planet discovered by Marcy and Butler (so far) has
less extreme attributes. Its three-year orbit takes it on a
circular course about 300 million kilometers from its star
(corresponding to an orbit between Mars and Jupiter) and its
mass is at minimum 2.3 times that of Jupiter. It would not seem
terribly out of place in our own solar system.

It comes as no surprise that astronomers are mostly finding
giant, short-period planets, for a simple reason: these are the
easiest to pick out using the Doppler technique. Detecting a
solar system like our own (in which the most massive planet,
Jupiter, takes a full 12 years to complete one orbit) would
require at least another decade or two of high-precision Doppler
observations.

In contrast, the astrometric approach that Gatewood uses is most
sensitive to planets in large, leisurely orbits. After
scrutinizing 50 years of observations of the star Lalande 21185,
he has tentatively deduced the presence of a Jupiter-mass planet
in a 5.8-year orbit; the planet would circle at more than twice
the earth's distance from the sun. Gatewood also sees evidence
of a second planet in a 30-year orbit. (He has released an
early abstract
describing these results.)

David C. Black of the Lunar and
Planetary Institute in Houston, Texas considers Gatewood's
sighting to be the only one that would satisfy his definition of
what a planet is. "It is not clear that any of the others have
anything to do with planets," he says, arguing that they
probably formed in a fundamentally different way than the
planets that orbit the sun.

Where Do Planets Come From?

Indeed, the question of how these planets (or non-planets)
formed is a vexing one that has already generated considerable
discussion. Current theory holds that giant planets can form
only at comparatively great distances from a star, where cold
temperatures allow ice and frozen gases to gather together. What
then are Jupiter-mass bodies doing so close to the stellar
hearth? Boss suggests that these planets actually formed at much
greater distances from their stars but then migrated inward.

One variant of this theory is described by Douglas N.C. Lin
of Lick Observatory and his colleagues in a recent issue of Nature. In this view, newborn
planets can interact with the disk of material from which they
form, causing them to spiral toward the central star. Inner
planets (which could have turned out to be more earth-like)
might have been destroyed during this early epoch. Whatever the
explanation, the surprising attributes of the new planets
clearly demonstrate that there are many ways that planetary
systems form

And what of planets like the earth--are they out there too? They
lie beyond the grasp of most current search techniques. But NASA
administrator
Daniel S. Goldin has set the detection and study of
earth-like bodies around other stars as one of NASA's top
priorities. The most likely way to achieve that goal is by
building a sophisticated space-based interferometer (as recently
described by J. Roger P. Angel and Neville J. Woolf in this
magazine). Ground-based tests of optical interferometers are
already underway.

The current list of extrasolar planets represents only the tip
of the iceberg. Continued observations, careful data analysis
and innovative technologies will soon yield many more
discoveries, giving us a better sense of the true variety of
worlds out there.